You're having a really good time, dressed up to the nines and feeling sophisticated when suddenly, you're hit by an unwelcome flashback.

Even A-listers like Jennifer Garner have this happen to them - and at events like the Oscars , no less.

Dressed in a stunning cobalt blue Atelier Versace gown, Garner was was applauding with the rest of the audience in the Dolby theatre, when her face suddenly underwent a dramatic transformation and she stopped clapping.

So what haunting realisation had hit her? Or had she simply remembered she'd left her hair-straighteners on?

There are some theories...

Garner's facial U-turn caught the attention of several delighted Oscars fans, who weighed in with what they thought had just occurred to her.

Garner' sudden and unpleasant lightbulb moment wasn't the only thing about the Alias actor's Oscar appearance which captured people's attention.

The mum-of-three looked absolutely amazing, and was sporting a dazzling amount of bling.

Her 18-carat white gold earrings set with diamonds and blue sapphires were worth, an eye-watering $605,000, according to PageSix.com .

She also flashed two Piaget rings, priced at $28,600 and $192,000.

(Image: AFP)

Garner's facial expression won't be the first time a celebrity clapping has got people amused.

It was Nicole Kidman's turn in 2017 as she was spotted clapping with zeal at the Oscars.

Kidman's bizarre technique was quick to become one of the talking points of last year's ceremony, but luckily there was simple explanation.